Inference of the worldwide invasion routes of the pinewood nematode Bursaphelenchus xylophilus using approximate Bayesian computation analysis

Population genetics have been greatly beneficial to improve knowledge about biological invasions. Model-based genetic inference methods, such as approximate Bayesiancomputation (ABC), have brought this improvement to a higher level and are now essential tools to decipher the invasion routes of any invasive species. In this paper, weperformed ABC random forest analyses to shed light on the pinewood nematode (PWN)worldwide invasion routes and to identify the source of European populations. Originating from North America, this microscopic worm has been invading Asia since 1905 andEurope since 1999, causing tremendous damage on pine forests. Using microsatellitedata, we demonstrated the existence of multiple introduction events in Japan (at leasttwo involving individuals originating from the USA) and China (one involving individualsoriginating from the USA and one involving individuals originating from Japan). We alsofound that Portuguese samples had a Japanese origin. We observed some discrepanciesbetween descriptive genetic methods and the ABC method, which are worth investigating and are discussed here. The ABC method helped clarify the worldwide history of thePWN invasion, even though the results still need to be considered with some cautionbecause the features of the PWN and the genetic markers used probably push the ABCmethod to its very limits.